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[TowerTalk] ++ High SWR and Modern Transceivers

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Subject: [TowerTalk] ++ High SWR and Modern Transceivers
From: W8JI@contesting.com (Tom Rauch)
Date: Tue, 11 Dec 2001 12:48:33 -0500
> less drive from the transceiver is available. As all other form of
> energy, SWR must go somewhere ... As it is not allowed to reach the
> transmitter output it will dissipate in the antenna tuner components
> as heat. If there is enough heat it might melt some components like my
> roller inductor in the MFJ -989C :)

Actually transmitters are energy conversion devices. Maximum 
available power capability that does not reach the antenna does not 
necessarily turn to heat. As a matter of fact a mismatch just as 
often reduces heat in the transmitter PA....but it does so at the 
expense of increased voltages in the PA.

If the mismatch is such that load impedance at the output device 
increases, heat is generally reduced. If the mismatch is in a 
direction that reduces load impedance seen by the PA output 
device, the dissipation generally increases.  

Despite common wisdom that professes SWR adjustment is 
important for improved tuner power ratings, tuners have a similar 
effect as PA stages. Heating generally relates to current through a 
component, and certainly does NOT depend on a match at the 
antenna. As a matter of fact, a mismatch can greatly reduce 
heating!

> Manufacturers of Antenna Tuners are specifying them for certain power
> levels ... 100W to more than 4KW. However, this is valid in a perfect
> resistive load, where there is no reactance. Once some reactance is
> added, like a real life antenna will do, the tuner specifications must
> be derated.

Not so. 

Consider the case of a conventional T network matching an 
impedance lower than the lowest obtainable surge impedance of 
the network. With 220pF capacitors, that would be about 3000 
ohms on 160 (1500 ohms on 80 meters, 750 ohms on 40 meters, 
and so on).

If I add inductive reactance to the load on a T-network, the power 
handling of the network actually increases. In some cases it can 
be a significant increase in power handling.

If I add series capacitive reactance, the power handling generally 
decreases.

With no change at all in load impedance, just by changing how I 
adjust the capacitors in a tuner, I can make a tuner that melts 
down or arcs at 100 watts safely handle a kilowatt or more! This is 
the reason it is VERY important to ALWAYS adjust your tuner so 
the capacitors are as far meshed as possible while still allowing a 
match, and why a better tuner design generally uses a large value 
of capacitance (lots of plates) over having wide plate spacing (and 
fewer plates) in a similar physical size capacitor. 

If you are buying a tuner by looking for wide plate spacing and high 
voltage rating, you are making a poor choice (unless the 
capacitance is also the same or higher)! 

The same tuner into exactly the same load can "melt down" if you 
use less than maximum available capacitance  match the load.

T-network tuners (on lower bands) also handle significantly more 
power into a higher impedance load than they do into low 
impedance loads, so the power rating at 3000 ohms load 
impedance (a 60:1 50-ohm normalized VSWR) can be many times 
better than the same tuner feeding a 50 ohm load!

How much "wasted power" does it take to melt a conventional 
inductor with #14 wire and plastic supports? About 10 to 50 watts 
of heat will do it. An insignificant amount of power loss, when 
running a kilowatt, will destroy most tank coils.

    
73, Tom W8JI
W8JI@contesting.com 

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